The present invention relates to a cylinder head gasket with partial seal coatings around the holes for the cylinder bores for securely sealing around the same.
As shown in FIG. 8, a cylinder head gasket 11 with a plurality of holes for the cylinder bores for sealing between a cylinder head and a cylinder block includes on a plate 20 holes 13 for the cylinder bores, oil holes 15, water holes 16, and bolt holes 17, 18. Also, in order to seal combustion gases generated in the combustion chambers, oil circulating through an oil gallery and water in a water jacket, the gasket 11 includes sealing means, such as beads 12 and seal rings 14, for providing adequate sealing pressures for the respective holes to be sealed.
Especially, since high temperature and high pressure combustion gases are generated around the holes 13 for the cylinder bores when the engine is operated, sealing around the holes 13 is especially important. Therefore, the metal plate 20 superior in mechanical strength and durability is used, and beads 12 for forming main sealing portions are arranged around the holes 13 for the cylinder bores to obtain high sealing characteristics by the beads 12.
Also, in the bolt holes 17, 18, head bolts (not shown) are inserted, and the gasket is sandwiched between the cylinder head and the cylinder block (not shown) and is tightened by the head bolts. The bolt holes 17, 18 are arranged to surround the holes 13 for the cylinder bores to securely seal around the cylinder bores.
Incidentally, the tightening pressures by the bolts passing through the bolt holes 17 are applied to the areas or ranges A in FIG. 8, while the tightening pressures by the bolts passing through the bolt holes 18 are applied to the areas B in FIG. 8. The surface pressures are formed by tightening the bolts.
In case the cylinder head gasket 11 in the prior art is installed between the cylinder head and the cylinder block, in case the sealing pressures by the tightening forces of the bolts are too weak, the function for sealing the combustion gas can not be sufficiently obtained. On the other hand, in case the sealing pressures by the tightening forces of the bolts are too strong, it causes the damages to the beads or creep relaxation to the beads. Thus, the sealing characteristics of the beads can not be maintained for a long period of time. Therefore, it is required to tighten the bolts at the adequate tightening pressures, so that the sealing pressures generated by the tightening pressures of the bolts can provide sufficient sealing characteristics for a long time.
For this purpose, when the cylinder head gasket 11 is installed on the cylinder block together with the cylinder head, the bolts are tightened according to the predetermined order. In addition, in order to provide the equal tightening torque by the bolts, the bolts are tightened by a torque wrench to the predetermined torque.
In the gasket 11 as shown in FIG. 8, the sealing area B to which the tightening forces of the bolts passing through the bolt holes 18 are applied is very small as compared to the area A to which the tightening forces of the bolts passing through the bolt holes 17 are applied. Therefore, the surface pressures acted on the area A become very small relative to the surface pressures acted on the area B.
On the other hand, since the engine is made smaller, the holes 13 for the cylinder bores are arranged as close as possible. Thus, the area W between the holes 13 for the cylinder bores comes a severe condition. Namely, in the area W, heat load is high, and combustions of gases different in timing are repeated in the adjacent cylinder bores. Therefore, the beads 12 are likely to cause creep relaxation and damages, so that gas leakage may happen.
In order to solve the problems, it may be thought that an auxiliary bead is formed adjacent to a main bead, and a stopper formed of a metal shim is provided to thereby prevent creep relaxation of the bead.
However, since the auxiliary bead and metal shim are disposed between the sealing portions of the two cylinder bores, it requires an additional area for installing these members. Since the engine has been made to become smaller, the area between the adjacent cylinder bores is made small. Therefore, it is difficult to arrange these members in the small area.
Also, in case the metal shim is used, the metal shim is very thin, i.e. generally 30-100 xcexcm, which is required to be processed precisely and sandwiched between plates or fixed to a plate. Therefore, there are troubles of taking time for processing the shim and storage to cause high cost.
Further, the metal shim does not have compressibility, and there is no thin metal shim less than 30 xcexcm. Therefore, in case the shim is used, there are steps at the ends of the shim to form large difference in surface pressures.
The present invention has been made to obviate these problems, and an object of the invention is to provide a cylinder head gasket, wherein the surface pressures of the beads situated between the holes for the adjacent cylinder bores are reinforced to improve the sealing characteristics.
Another object of the invention is to provide a cylinder head gasket as stated above, wherein creep relaxation and damage of the beads are prevented to enhance the durability of the gasket.
Further objects and advantages of the invention are apparent from the following description of the invention.
A cylinder head gasket of the invention is used for an internal combustion engine with a plurality of cylinder bores. The gasket is formed of a first metal plate extending substantially throughout an entire area to be sealed. The first metal plate includes a plurality of holes corresponding to the cylinder bores of the engine, a plurality of beads surrounding the respective holes for the cylinder bores, and at least one area situated between the holes adjacent to each other. Seal coatings are deposited above the beads in the at least one area. The seal coating has heatproof characteristic, i.e. strong against heat, and compressibility to strengthen the surface pressure of the beads at the at least one area.
The seal coating may be selected from the group consisting of epoxy resin, phenol resin, phenoxy resin, fluoroplastics and polyamideimido. A desired shape of the seal coating may be applied by screen printing or spraying with a mask. The seal coating may have one layer or a plurality of layers. Especially, phenoxy resin has heat resistance over 200xc2x0 C., and in view of working ability and coating characteristics, phenoxy resin is most preferable.
In the engine having the cylinder bores more than two and arranged linearly, the gasket has the coatings with heat resistance, which are applied to the area between the adjacent holes for the cylinder bores to be subjected to the severe condition where the tightening strength is low. The surface pressures of the beads applied to the area between the holes for the cylinder bores can be reinforced by the coatings with the compressibility. In this reinforcement of the surface pressures, the sealing pressures are increased to have the pressures substantially the same as those in the end areas of the holes for the cylinder bores. Therefore, the sealing can be made securely.
Also, in this coating with compressibility, reaction forces of the cylinder head and the surface pressures applied to the beads between the holes for the cylinder bores are reduced or relieved, so that the creep relaxation and damage of the beads to which the coatings are applied are prevented.
In the gasket, an adequate sealing pressure can be formed at the inner cylinder by the adequate sealing pressures for the cylinders at the longitudinal ends of the engine. Thus, it does not require a large tightening torque, so that the creep relaxation and damage of the beads are prevented.
When the coatings are made by a screen printing, even if the coatings have complicated shapes, the coatings can be made easily. Further, the thickness of the coating can be changed easily by multiple printings, and the coating with an adequate thickness can be formed easily.
Incidentally, the gasket of the invention may be formed of one or a plurality of metal plates. Also, in addition to the above coating, the gasket may have another coating, such as seal coating formed at an outer periphery of the gasket.
In the invention, the seal coatings deposited above the beads in one area are integrally connected together as one unit. In this case, the seal coatings may be entirely deposited on the first metal plate between the beads. When the gasket is formed of one metal plate, the seal coatings are applied on the metal plate. However, when another metal plate is disposed on the metal plate at a side where the beads project, the seal coatings may be applied on the another metal plate facing the beads.